The present invention relates to a power transmission unit for automotive vehicles, and more particularly to an air breather adapted to a casing assembly for the power transmission unit.
In such a casing assembly for the power transmission unit, a rotary shaft extends outwardly from the casing assembly through oil seal members, and an air breather is provided to allow the flow of air out from the interior of the casing assembly to the exterior so as to prevent an increase of the pressure in the casing assembly upon operation of the unit and to eliminate leakage of the lubricating oil out from the casing assembly across the oil seal members. In a conventional transmission unit, an air passage is formed in a fitted portion of the casing assembly to a cover member fixed thereto in a fluid-tight manner to provide an air breather at a low cost. It has, however, been observed that if the air passage is inappropriate in its position and configuration, there will occur leakage of the lubricating oil across the air passage, causing unexpected difficulty in actual practice. Such leakage of the lubricating oil is caused by formation of an oil film across the air passage, which permits the flow of oil out from the casing assembly under the increased pressure therein.